SCP-controlled overlay between GSM and IMS

ABSTRACT

The invention relates to method of executing a service in a Media Gateway Control Function ( 12 ) for establishing a call between a calling party&#39;s terminal ( 8 ) served by a Mobile Switching Center ( 2 ), and an IP Multimedia Subsystem Service ( 20 ). The Media Gateway Control Function ( 12 ) is arranged to query a Service Control Point ( 10 ) on which an overlay process is loaded. The correct SCP ( 10 ) is identified by the MGCF ( 12 ) using a correlation identifier originally published by the SCP itself after having received an IN service trigger from an MSC or GMSC. The MGCF will receive call related data from the SCP, such as the original dialled number. This data is used by the MGCF to populate information elements in a SIP Invite message. The invention gives a solution for providing GSM specific information, e.g. dialled number, location information, to an IMS service. This information may not be available to the IMS service through present methods.

TECHNICAL FIELD

The present invention relates to a method of routing a call to an IMSservice.

BACKGROUND

Registration Surrogate (RS) is a technique that enables an operator toregister GSM subscribers or ISDN subscribers in an Internet ProtocolMultimedia Subsystem (IMS) network. The registered IMS subscribers canuse GSM access, but have their services reside in the IMS network. TheRS facilitates triggering of one or more IMS services when a callto/from such a GSM subscriber is routed from a GSM network to an IMSnetwork. The routing of a call from GSM to IMS may be performed usingthe well known Customized Application of Mobile Network Enhanced Logic(CAMEL) “Route Home” service. The routing of a GSM call through IMS, forthe purpose of executing one or more IMS services in the IMS network, isknown as “GSM-IMS overlay”.

One example of an IMS service for which it is desirable to be accessiblein the above mentioned manner is IP-Centrex. IP-Centrex is an IMSservice offering Wireless Office features. One of the aspects ofIP-Centrex is the possibility to access an IMS service with GSMterminals, as well as with Session Initiation Protocol (SIP) phones.Such IMS service is commonly referred to as Multi Access IMS service.The remainder of the description takes IP-Centrex as example IMSservice. The invention is, however, equally applicable to any other IMSservice that may be accessed through GSM, such as Call Completion orIncoming Call Screening. Furthermore, the invention is equallyapplicable to calls in fixed telecommunication networks, as in mobilenetworks such as GSM networks or UMTS networks.

When applying CAMEL for the overlay between GSM and IMS, the calledparty number (CdPN), as dialled by the calling subscriber in the case ofcalls originating in the GSM network, can not always be maintained inthe ISDN User Part (ISUP) signalling between a GSM network and an IMSnetwork. This has mainly three reasons.

Firstly, the CdPN that is carried in the ISUP Initial Address Message(IAM) is limited to 15 address digits, by ITU-T recommendation E.164.But 15 digits may not be sufficient to address a Media Gateway ControlFunction (MGCF) and also includes the original dialled number,especially when the GSM call is established from abroad or from anetwork of a different provider. Secondly, the number frame of the CdPNin ISUP contains a Nature of Address (NoA). When an MGCF address,required for routing the call to the MGCF, is added to the CdPN, the NoAin the frame of the ISUP CdPN has to be set to national orinternational, depending on whether the GSM call is established in thehome country or from abroad or from a different provider. As a result,the NoA of the original dialled number gets lost. A third reason is thatthe user may include operator-specific service selection codes (* and #)in the dialled number. These codes can not be conveyed, through ISUP, tothe IMS service.

Also, when ISUP signalling traverses (inter)national network boundary,the Calling party number (CgPN) may get corrupted or lost. The CgPN ishowever needed to identify the calling subscriber and to trigger the IMSservice. Hence, when the CgPN is not available or when it is corrupted,there is no IMS triggering possible and hence, the call can not beestablished through IMS overlay.

Finally, when the IMS service is triggered, vital information such asLocation Information is not available to the IMS service.

SUMMARY OF THE INVENTION

A goal of the present invention is to provide a method for establishinga call between a calling party's terminal (e.g. fixed phone in a PSTN orISDN, or a mobile phone in a PLMN) served by a Switching node and an IMSservice that overcomes one or more of the problems mentioned above.

This goal is achieved by providing a method of executing a service in aMedia Gateway Control Function for establishing a call between a callingparty's terminal served by a Switching Node, and an IP MultimediaSubsystem service, the method comprising:

-   -   receiving a call initiation message from the Switching Node, the        call initiation message comprising a called party number;    -   analysing the called party number to see whether the call set up        is requested for a connection to the IP Multimedia Subsystem        service;    -   in the case that the call is a connection to said IP Multimedia        Subsystem service, deriving from the called party number which        Service Control Point is in charge;    -   querying said Service Control Point for data associated with the        call;    -   processing the data in order to create a Session Initiation        Protocol Invite message;    -   sending the Session Initiation Protocol Invite message to a Call        Session Control Function.

The Call Session Control Function will then further execute the callestablishment process.

In an embodiment, the called party number comprises:

-   -   a country code of an operator of said IP Multimedia Subsystem        Service;    -   a national destination code for said operator;    -   an address of said Media Gateway Control Function;    -   a correlation identifier identifying an overlay process on the        Service Control Point.

The data comprises one or more of the following:

-   -   an original dialled number dialled at said calling party's        terminal;    -   a calling party number associated with said call;    -   location information of said calling party;    -   a called party number associated with said called party;    -   location information of a called party associated with said        called party number;    -   an International Mobile Subscriber Identification;    -   an International Mobile Equipment Identity;    -   a subscriber state associated with said called party.

According to an aspect of the invention, there is provided a MediaGateway Control Function for use in a telecommunication network, whichMedia Gateway Control Function is arranged to perform the abovementioned method.

According to another aspect, there is provided a telecommunicationnetwork comprising at least one Media Gateway Control Function asdescribed above.

According to yet another aspect, there is provided a method of executinga service in a Service Control Point for establishing a call between acalling party's terminal served by a Switching Node and an IP MultimediaService, the method comprising:

-   -   receiving an IN service trigger from the Switching Node, the IN        service trigger comprising data associated with the call;    -   storing the data;    -   starting up an overlay process for the call;    -   allocating a correlation identifier identifying the overlay        process;    -   sending a destination subscriber number to the Switching Node;    -   on receiving a request from a Media Gateway Control Function,        providing the data to the Media Gateway Control Function.

The invention also relates to a Service Control Point for use in atelecommunication network, the Service Control Point being arranged toperform the method described above.

Finally, the invention relates to a telecommunication network comprisingat least a Service Control Point as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed in more detail below, using anumber of exemplary embodiments, with reference to the attacheddrawings, in which:

FIG. 1 shows a network diagram which depicts GSM-IMS overlay for amobile originated call according to the state of the art;

FIG. 2 shows a network diagram which depicts GSM-IMS overlay for amobile originated call according to an embodiment of the presentinvention;

FIG. 3 depicts a network diagram which depicts GSM-IMS overlay for amobile terminating call according an embodiment;

FIG. 4 depicts a network diagram of the network of FIG. 2 in which acall is established using an URI instead of a number;

FIG. 5 is an example of a correlation identifier;

FIG. 6 is a flow chart of actions taken by an MGCF according to anembodiment in order to establish a call to a GSM MAE subscriber;

FIG. 7 is a flow chart of actions taken by an SCP according to anembodiment.

DETAILED DESCRIPTION

FIG. 1 depicts a GSM network 9 together with (part of) an IMS network 11according to the state of the art. In FIG. 1, functional blocks aredepicted, each of which may be incorporated is a separate network node,but some of them may as well be combined into a single network node. TheGSM network 9 comprises a serving Mobile Switching Centre (MSC) 2 and aHome Location Register (HLR) 4 which communicates with the serving MSC 2via e.g. a SS7 network (not shown). Furthermore a Radio Access Network(RAN) 6 is shown via which a calling party may set up a call through theGSM network 9 using a mobile station, such as a mobile phone 8. Theserving MSC 2 is also communicating with a SCP 10 using CAMELApplication Part (CAP) as will be known by the skilled person. In theexample of FIG. 1, the GSM network 9 is a Visited Public Land MobileNetwork (VPLMN) 9. The GSM network 9 is in communication with the IMSnetwork 11 which comprises a MGCF 12, a P-CSCF 14, a S-CSCF 16 and anIMS service 20. The P-CSCF 14 is the first contact point in the IMSnetwork and it provides authorization of bearer resources. It alsoforwards a SIP register request received from a SIP User Agent (notshown) to an I-CSCF (not shown) determined using the home domain name,as provided by the IMS subscriber. In the case of Registration Surrogate(RS) the P-CSCF 14 receives the SIP register request from the RS. In theopposite direction it forwards the SIP request or response to a SIPPhone (not shown). In the case of RS, it forwards the SIP request orresponse to the MGCF 12. Moreover the P-CSCF 14 forwards SIP messagesreceived from the MGCF 12 to the S-CSCF 16.

Now, a typical call flow for a Mobile originated call establishment froma GSM subscriber having access to the IMS is described according to thestate of the art. This GSM subscriber is also referred to as a GSM MultiAccess Extension (MAE) subscriber. The subscriber registers in the MSC 2in the VPLMN 9, see FIG. 1. The subscriber has a profile in the HLR 4for Centrex functionality. This profile consists of an IN servicetrigger named Originating CAMEL Subscription Information (O-CSI). TheHLR 4 sends the O-CSI to the MSC 2. The GSM MAE Subscriber initiates aMobile Originated call. The subscriber may dial a short code (e.g. 4500)or a public number (e.g. number in international format +31 161 24 9911;or a number in unknown format 00 31 161 24 9911; 0161 24 9911). Sincethe GSM MAE subscriber has O-CSI, the MSC 2 triggers an IN service inthe SCP 10, i.e. the MSC 2 sends a CAMEL Application Part (CAP) InitialDetection Point (IDP) Operation to the SCP 10.

A CAP dialogue is established between the MSC 2 and the SCP 10. The SCP10 resides in the home network of the GSM subscriber. The operator thatoperates this home network, known as Home Public Land Mobile Network(HPLMN), may also be the operator that operates the IMS network. Theaddress of the SCP 10 is derived from the O-CSI. Dynamic load sharingmay be applied to the IN service triggering.

The IN service may be the CAMEL Route Home service. It is noted that“CAMEL Route Home” is not official GSM terminology, but is commonly usedfor a CAMEL service that routes calls from a GSM subscriber to an entityin the home network. For the purpose of GSM-IMS overlay, the CAMEL RouteHome service routes the call to the IMS network 11, so the service isalso referred to as “CAMEL Route to IMS” service. The IN service insertsa prefix in front of the original dialled number from the subscriber.The prefix results in routing the call to the MGCF 12 of the IMS network11. It should be noted that the CAMEL Route to IMS service may bedeployed on two or more SCPs. An SS7 network (not shown) connecting theMSC 2 and the SCP 10 will route the service invocation to the SCP 10.

The SCP 10 determines that the subscriber is a GSM MAE subscriber (byvirtue of a service key in the CAP IDP) and gives the MSC 2 aninstruction to route the call to the MGCF 12 in the IMS network 11. TheIMS network 11 contains one or more MGCFs. The number of MGCFs isdetermined by, amongst others, the number of subscribers in the IMSnetwork 11 and by the geographical size of the IMS network 11. The SCP10 will determine which MGCF to route the call to, e.g. based on theservice key.

The Called Party Binary Coded Decimal (BCD) Number (CdPBN) is sent fromthe mobile phone 8 to the MSC 2, over the RAN 6. It contains the numberas dialled by the subscriber (i.e. entered on the keypad of the GSMphone). The MSC 2 converts the CdPBN to a Called Party Number (CdPN).Further call handling in the GSM network 9, i.e. routing the call to thedestination of this call (called party), is done with this CdPN. The SCP10 will provide a different CdPN for this call, i.e. it overrides theCdPN that the MSC 2 had derived from the CdPBN, by a different numberreferred to as ‘destination subscriber number’. The SCP 10 provides thisdestination subscriber number to the MSC 2 in a CAP Connect operation.For the purpose of GSM-IMS overlay, the destination subscriber numberhas the following format:

-   -   <cc> <ndc> <mgcf address> <original dialled number>    -   whereby    -   <cc>: country code of the IMS operator    -   <ndc>: national destination code for the IMS operator    -   <mgcf address>: address of the MGCF 12    -   <original dialled number>: sequence of numbers dialled by the        calling party

The <mgcf address> consists of a number of digits to identify the MGCF12 within the IMS network 11. The concatenation of <cc>, <ndc> and <mgcfaddress>constitutes the aforementioned “prefix”. The SCP 10 inserts theprefix in front of the original dialled number and returns the result tothe MSC 2.

The MSC 2 routes the call to the MGCF 12 of the IMS network 11. Therouting of the call to the MGCF 12 is done in accordance with standardISUP routing methods. The Called Party Number (CdPN) in the ISUP IAMcontains the destination subscriber number that was provided by the SCP10 in the CAP Connect operation. The MGCF 12 will then receive the ISUPIAM and applies ISUP to SIP conversion. The MGCF 12 generates a SIPInvite message and sends the SIP Invite to the P-CSCF 14. By using adesignated IP port address, the SIP Invite at the P-CSCF 14 will beinterpreted as User-to-Network Interface (UNI) signalling.Alternatively, this SIP Invite may be sent to a dedicated P-CSCF usedfor GSM-IMS overlay.

The GSM MAE subscriber is registered in the P-CSCF 14. The GSM MAEsubscriber is also registered in the S-CSCF 16. The registration in theP-CSCF 14 and S-CSCF 16 is performed by the Registration Surrogate. TheP-CSCF 14 forwards the SIP Invite to the S-CSCF 16 where this subscriberis registered. The S-CSCF 14 has so-called Initial Filter Criteria (IFC)stored for this subscriber. It will use the IFC to trigger theIP-Centrex service. The Request Universal Resource Identifier (RequestURI, R-URI) in the SIP Invite contains the original dialled number. TheIMS service handles the call by this subscriber, depending on thesubscriber's profile in the IMS service.

When applying overlay between GSM and IMS, the original dialled numbercan not always be maintained in the ISDN User Part (ISUP) signallingbetween the GSM network 9 and the IMS network 11. This is caused by thefact that the CdPN in the ISUP IAM is limited to a predetermined numberof address digits (e.g. in ITU-T recommendation E.164, the number is15).

According to an embodiment of the present invention, this problem issolved by letting the SCP 10 and the MGCF 12 communicate with each otherso that the MGCF 12 can retrieve the original dialled number from theSCP 10 during call establishment, see FIG. 2. FIG. 2 depicts a typicalcall flow for a Mobile Originated call establishment from the GSMnetwork 9, including an enhancement described in the present invention.

In FIG. 2 corresponding reference numbers are used for correspondingfunctional blocks of FIG. 1. According to this embodiment, the SCP 10 isarranged to store the information it has received from the MSC 2 in theCAP IDP. This information may include, the CdPBN, the CgPN, anInternational Mobile Subscriber Identification (IMSI), an InternationalMobile Equipment Identity (IMEI) and Location Information.

In a particular embodiment of the present invention, the SCP 10 and theMGCF 12 are arranged to communicate the CgPN via an IP based networkusing the SOAP protocol to communicate XML messages (referred to asSOAP/XML query), see FIG. 2. If the CgPN gets lost in the ISUPsignalling from the MSC 2 (in the VPLMN 9) to the MGCF 12 (in the IMSnetwork 11), then the CgPN will be restored through a SOAP/XML queryfrom the MGCF. In an embodiment, the SCP 10 is arranged to provide adestination subscriber number in a CAP Connect (CON) operation which hasthe following format:

-   -   <cc> <ndc> <mgcf address> <correlation identifier>    -   whereby    -   <cc>: country code of the IMS operator    -   <ndc>: national destination code for the IMS operator    -   <mgcf address>: address of the MGCF 12    -   <correlation identifier>: a process identifier of a GSM-IMS        overlay process on the SCP 10.

Please note that the original dialled number is not part of thisdestination subscriber number as was the situation in the SCP accordingto the state of the art. The correlation identifier is allocated by theSCP 10. The SCP 10 has a pool (i.e. a list) of correlation identifiers.If there are multiple SCPs for this CAMEL Route to IMS service, theneach SCP 10 has a dedicated range of correlation identifiers.

The correlation identifier may contain over-decadic digits. The use ofover-decadic digits prevents that GSM users (or PSTN users) can manuallydial a correlation identifier.

Once the SCP 10 has connected the call, it will release the CAP dialoguewith the MSC 2. The MSC 2 routes the call to the MGCF 12 in the IMSnetwork 11. The routing of the call to the MGCF 12 is done in accordancewith standard ISUP routing method. The MSC 2 sends an ISUP IAM to theMGCF 12. The IAM contains the parameter CdPN. The MSC 2 copies thecontent of the destination subscriber number from CON into the parameterCdPN in the ISUP IAM. Hence, the CdPN in the ISUP IAM now contains theprefix plus the correlation identifier. The combination of the mgcfaddress and the correlation identifier indicates to the MGCF 12 thatthis call is a GSM multi access call. The MGCF 12 is arranged to store alist comprising possible combinations of the mgcf address and thecorrelation identifier. Said list may consist of one or more numberranges; each number range may consist of the mgcf address plus a rangeof correlation numbers. When an ISUP IAM is received, the MGCF 12 willlook in the list and if it finds a matching combination, it willconclude that this is a call from a GSM MAE subscriber. Now, the MGCF 12shall in response contact the SCP 10. The SCP address is derived fromthe correlation identifier. Since the SCP 10 and the MGCF 12 areoperated by the same operator, the SCP address(es) can be configured inthe MGCF 12. After having derived the SCP address, the MGCF 12 usesSOAP/XML to query the SCP 10. The MGCF 12 includes the correlationidentifier in the SOAP/XML query. In an embodiment, the MGCF 12 isarranged to obtain data from the SCP 10, using SOAP to do a remoteprocedure call towards the SCP 10. The query towards the SCP 10 (e.g.‘provide call data’), the parameter for this remote call (e.g.‘correlation identifier’) and the return parameters (e.g. ‘originaldialled number’, ‘calling party number’ etc.) are represented in XML.

The SCP 10 uses the correlation identifier to retrieve the stored callinformation. The SCP 10 returns the data to the MGCF 12. After that, theSCP 10 purges the stored data and returns the correlation identifier tothe pool. The correlation identifier may now be re-used.

The correlation identifier is used only for the duration of call routingbetween MSC 2 and MGCF 12 and the interrogation from the MGCF 12 to theSCP 10. Hence, a small number of correlation identifiers may suffice.

The SCP 10 will also supply the address of the P-CSCF 14 to the MGCF 12.The GSM MAE subscriber is registered in one of a number of P-CSCFs. TheSCP 10 may be configured with the P-CSCF address for the subscriber, orthe P-CSCF address may be derived from the service key. The SCP 10 mayhave received the P-CSCF address from the Registration Surrogate (RS)after the RS had registered the subscriber in the IMS network 11.

Next, the MGCF 12 applies an ISUP to SIP conversion. The MGCF 12generates a SIP Invite and populates the SIP Invite with the informationthat it has received in the SOAP/XML response, such as original diallednumber and CgPN. The MGCF 12 sends the SIP Invite to the P-CSCF 14. Byusing a designated IP port address, the SIP Invite at the P-CSCF 14 willbe interpreted as User-to-Network Interface (UNI) signalling. The IMSnetwork 11 then handles the call as was described above with referenceto FIG. 1.

Thanks to the communication between the MGCF 12 and the SCP 10, therewill be no loss of information in the CgPN.

The signalling link between User and P-CSCF may be regarded as UNI, forthe purpose of connecting the GSM subscriber to the IMS network. Oncethe call has arrived at the S-CSCF 16 of the IMS network 11, the callwill be handled in accordance with the user's profile.

In an embodiment, the triggering of the CAMEL route to IMS service maypass through a Service Capability Integration Manager (SCIM) situatedbetween the MSC 2 and the SCP 10. Likewise, the ISC interface betweenS-CSCF 16 and the IP-Centrex may pass through a SCIM (not shown). TheSCIM is arranged to call and integrate a plurality of services using asingle trigger from the GSM network 9 or from the IMS network 11.

The CAMEL Route to IMS service and the IMS service have no correlationwith one another. Hence, the IMS service is not dependent on access tothe SCP 10 in the GSM network 9. The required information is availablein the SIP Invite coming from the S-CSCF 16. In this manner, other IMSservices may use the Location Information and IMSI and otherGSM-specific data in the SIP Invite.

Table 1 provides an overview of the GSM access specific information thatis placed in the SIP Invite sent by the MGCF 12 to the IMS network 11according to an embodiment.

TABLE 1 GSM element SIP Invite header Calling Party P-Asserted-IdentityNumber Called Party Request-URI BCD Number LocationP-Access-Network-Info; see 3GPP TS 24.229 for Information mappingbetween Location Information and P-Access-Network-Info. IMSI Pivate useridentity; 3GPP TS 23.003 specifies how the Pivate user identity may bederived from IMSI. Time and Timestamp; RFC 3261 defines how Time andTime Time zone zone from the Serving MSC may be converted to the Dateformat and the Timestamp format for SIP. MCC + MNC P-Visited-Network-ID;the SCP may apply mapping between the MCC + MNC, as received in CAP IDP,to a text string representing the GSM operator. MSC AddressP-Access-Network-Info; see RFC 3455 for a description of this SIPheader. It may be included in SIP Register, SIP Invite and other SIPmethods. Call Reference Call-Id; the Call-Id identifies a single call.Copying Number the GSM Call Reference Number into this field has theadvantage that CDRs that are generated in the GSM network may becorrelated with charging function in the IMS network. IMEI The IMEI ofthe calling subscriber is reported to the SCP in CAP IDP. This parameteris mapped on a SIP header. MS Classmark The MS Classmark of the callingsubscriber is reported to the SCP in CAP IDP. This parameter is mappedon a SIP header.

Now, an example of the sequence of events for a Mobile Terminated callestablishment to a GSM MAE subscriber will be discussed with referenceto FIG. 3. A call arrives at a Gateway MSC (GMSC) 30 for the GSM MAEsubscriber. The routing of the call to the GMSC 30 follows GSMprinciples as will be known to the skilled person. The GSM MAEsubscriber may e.g. be addressed with MSISDN.

The GMSC 30 contacts the HLR 34 to obtain routing information for thiscall. The subscriber is registered as GSM MAE subscriber in the HLR 34,i.e. the HLR 34 has terminating IN trigger information for thesubscriber. This information consists of CAMEL subscription information(T-CSI). An optional part of terminating call handling in the HLR 34 forsubscribers with CAMEL information is the obtaining of the subscriber'slocation information and state. Hereto, the HLR 34 contacts a VisitorLocation Register (VLR) where the subscriber is currently registered.

The HLR 34 returns the IN service subscription information (T-CSI) andthe subscriber's location and state information, if obtained from theVLR, to the GMSC 30. In this example, the GMSC 30 uses the T-CSI toinvoke a CAMEL Route to IMS service on an SCP 36. Hereto, the GMSC 30sends a CAP IDP to the SCP 36. The CAP IDP contains, amongst others, theCalled Party Number, Calling Party Number (if available), IMSI andLocation Information (if available).

The SCP 36 is arranged to store the information received in the CAP IDPand allocate a correlation identifier. For a description and use of thecorrelation identifier, reference is made to the above description ofthe Mobile originated call, see FIG. 2.

The SCP 36 routes the call to an MGCF 38 of an IMS network 40, bysending CAP Connect to the GMSC 30. The CAP Connect contains the addressof the MGCF 38 and the correlation identifier. The GMSC 30 routes thecall to the MGCF 38, using the information received in the CAP Connectoperation. The MGCF 38 deduces from the <mgcf address> and the<correlation identifier> in the Called Party Number in the ISUP IAM thatthis call is a call to a GSM MAE subscriber. Therefore, the MGCF 38 usesSOAP/XML to query the SCP 36. The address of the SCP 36 is derived fromthe correlation identifier.

The SCP 36 uses the correlation identifier to retrieve the stored data.The SCP 36 sends the original called number to the MGCF 38. In addition,the SCP 36 provides the called party's location information to the MGCF38. The calling party number was also stored in the SCP 36, but thisnumber will normally remain in the ISUP signalling from the GMSC 30 tothe MGCF 38, because the GMSC 30 and the MGCF 38 are located in the samenetwork. So, the calling party number does not need to be restored.However, if the GMSC 30 is located in another network than the MGCF 38,the calling party number may get lost in the ISUP signalling between theGMSC 30 and the MGCF 38. Therefore, according to an embodiment, the SCP36 is arranged to also provide the calling party number to the MGCF 38.

In an embodiment, a designated SIP header is defined for carrying theLocation Information of the called subscriber. The P-Access-Network-Infostrictly applies to the calling party. In addition, the SCP 36 mayprovide the following information related to the called party:

-   -   IMSI, and    -   Subscriber state.

For these parameters designated headers in a SIP Invite message may bedefined.

The MGCF 38 will now have all information to decide that the call isdestined for a GSM MAE subscriber. The MGCF 38 can therefore generate aSIP Invite message and send the SIP Invite message to a predefinedI-CSCF 42. The MGCF 38 includes the called party's location information,and other information elements such as IMSI and Subscriber state, in theSIP Invite message.

The I-CSCF 42 interrogates a Home Subscriber System (HSS) 44 to obtainthe called subscriber address of a S-CSCF 46. The subscriber isregistered in the IMS network 40, hence the HSS 44 has the S-CSCFaddress for this subscriber.

When the I-CSCF has received the S-CSCF address from the HSS 44, itroutes the call to the S-CSCF 46, i.e. sends the SIP Invite to theS-CSCF 46. The subscriber is registered in the S-CSCF 46, hence theS-CSCF 46 has Initial Filter Criteria (IFC) available for thissubscriber. The S-CSCF 46 will now invoke a corresponding IMS service50. In the current example, the IMS service 50 is a multi access IMSservice.

The multi access IMS service handles the call to this subscriber,depending on the subscriber's profile in the multi access service. Thetriggering of the CAMEL Route to IMS service as well as the IMS servicetriggering may pass through a SCIM (not shown) as was described withreference to FIG. 2. The multi access IMS service is not dependent onaccess to the SCP 36 in the GSM network. The required information,notably the called party's location, is available in the SIP Invitemessage coming from the MGCF 38. In this manner, other IMS services,such as charging, Voice Call Continuity and Multimedia Telephony, mayalso use the Location Information (and other information elements suchas IMSI) in the SIP Invite message. This enables the IMS service 50 toperform e.g. terminating call service functionality that needs a calledparty's location.

It is noted that the invention is not limited to the use of mobilenetworks, and that the invention also applies to fixed telephonenetworks, such as PSTN or ISDN. In that case the MSC 2 will be replacedby a Switching Node of a fixed telecommunication network, and thecallings party's mobile station 8 by a calling party's fixed terminal.

FIG. 4 shows the communication network of FIG. 2, in which correspondingreference numbers refer to corresponding functional blocks. In FIG. 4,with a call establishment by a GSM subscriber according to an embodimentis depicted using a Universal Resource Identifier (URI). In FIG. 4, theURI “matsson@telia.se” is used as the CdPBN. In this embodiment, theconcept of having the MGCF 12 obtain the original dialled number fromthe SCP 10, is used in combination with a method by means of which a GSMsubscriber establishes a mobile originated call, using a URI instead ofa number. E.g. set up a call to matsson@telia.se.

In an embodiment, the URI that is used by the calling GSM subscriber toestablish the call is provided by the SCP 10 to the MGCF 12. In thatmanner, the GSM subscriber can use SIP URI calling, even when the GSMcall is routed to the IMS network 11 for further processing.

In FIG. 5, an example is given of a correlation identifier identifyingthe GSM-IMS-overlay process on the SCP 10. The correlation identifiercomprises 5 digits, four of which define the GSM-IMS overlay process onthe SCP 10. The first digit is used to define the address of the SCP. Inthis example, the SCP 10 having an address “A” deals with the call. Andprocess “46B7” is in charge of the routing of the call with the MGCF 12.

FIG. 6 is a flow chart of actions taken by the MGCF 12 according to anembodiment in order to establish a call from a GSM MAE subscriber. In afirst step 61, the MGCF 12 receives a call initiation message from theMSC 2 comprising a called party number. This may be an ISUP IAM asdescribed above. The CdPN in the ISUP IAM is interpreted by the MGCF 12in a step 62. In a step 63, it is tested whether the call is a call toan IMS service. This testing is done by looking at the combination ofthe mgcf address and the correlation identifier as described above. Ifthe conclusion is NO, then normal call handling is continued, see step68. If the conclusion is YES it is a call from a GSM MAE subscriber, astep 64 follows in which the MGCD 12 derives from the CdPN which SCP isin charge. In other words, which SCP has allocated the correlationidentifier and runs an overlay process for this call. Since thedestination subscriber number is contained in the CdPN, the MGCF 12 isable to derive the correlation identifier. The correlation identifier isthen used to query the SCP 10 using for example SOAP/XML. Alternatively,the MGCF 12 may use Lightweight Directory Access Protocol (LDAP) toquery the SCP 10 or any other suitable protocol known to the skilledperson. The SOAP protocol may be used in order to communicate theSOAP/XML query to the SCP 10 and to communicate a SOAP/XML response tothe MGCF. The SCP 10 is queried for data associated with the call, seestep 65. This data was stored by the SCP 10 when it received the INtrigger from the MSC 2. Next in a step 66, a SIP Invite message iscreated using the data received from the SCP 10. Finally, the SIP Invitemessage is sent to the CSCF 14, see step 67.

FIG. 7 is a flow chart of actions taken by the SCP 10 according to anembodiment. In a first step 71, the SCP 10 receives an IN trigger fromthe MSC 2. The IN trigger comprises data such as the original diallednumber, which is stored by the SCP 10 in a memory of the network node onwhich the SCP 10 functions, see step 72. It is noted that the data maybe stored on several SCPs, as will be appreciated by the skilled person.Next, in a step 73, an overlay process is started for the call. Thisoverlay process will run on the SCP 10 until the MGCF 12 has receivedthe call related data from the SCP 10. The SCP 10 will also allocate acorrelation identifier for the call, see step 74. The correlationidentifier identifies the overlay process as was discussed withreference to FIG. 5. Next, the SCP 10 sends a destination subscribernumber to the MSC 2. This is done in the CAP Connect operation. If arequest is received from an MGCF 12, see step 76, the overlay processrunning on the SCP 10 will deal with the request. The overlay processwill access the memory on which the call related data was stored. Thisdata is then provided to the requesting MGCF 12, which will use it torestore the information it received from the MSC 2.

It is noted that the SCP 10, 36 may constitute a service executionplatform embodied as a single node or comprised as a functional entitywithin another node.

The present invention has been explained above with reference to anumber of exemplary embodiments. As will be apparent to the personskilled in the art, various modifications and amendments can be madewithout departing from the scope of the present invention, as defined inthe appended claims. Instead of using GSM, UMTS may be used as will beappreciated by the skilled person. Also it is noted that other INservices may be used and that the invention is not limited to the use ofCAMEL. Furthermore, it is noted that the invention can also be used in acall establishment from a fixed telecommunication network to an IMSservice.

List Of Abbreviations

CAMEL Customized Application of Mobile Network Enhanced Logic CAP CAMELApplication Part CdPBN Called Party Binary coded decimal Number CdPNCalled Party Number CDR Call Detail Record CgPN Calling Party NumberCSCF Call Session Control Function CSI Combination of CS and IMSservices or CAMEL Subscription Information GMSC Gateway MSC HPLMN HomePLMN HSS Home Subscriber System IAM Initial Address Message IMS IPMultimedia Subsystem IP Internet Protocol IPLMN Interrogating PLMN ISCIMS Service Control interface ISDN Integrated Services Digital NetworkISUP ISDN User Part I-CSCF Interrogating CSCF MGCF Media Gateway ControlFunction MS Mobile Station MSC Mobile Switching Centre O-CSI OriginatingCAMEL Subscription Information PLMN Public Land Mobile Network PS PacketSwitched PSTN Public Switched Telephone/Telecommunications NetworkP-CSCF Proxy CSCF RAN Radio Access Network R-URI request URI SCIMService Capability Integration Manager SCP Service Control Point SIPSession Initiation Protocol SIP-URI SIP Universal Resource IdentifierSOAP Simple Object Access Protocol S-CSCF Serving CSCF SR SurrogateRegistration T-CSI Terminating CAMEL Subscription Information UNIUser-to-Network Interface URI Universal Resource Identifier UTRANUniversal Terrestrial Radio Access Network VPLMN Visited PLMN XMLExtended Mark-up Language

1. A method of executing a service in a Media Gateway Control Functionfor establishing a call between a calling party's terminal served by aSwitching Node, and an IP Multimedia Subsystem Service, said methodcomprising: receiving a call initiation message from said SwitchingNode, said call initiation message comprising a called party number;analyzing said called party number to determine whether the call set upis requested for a connection to said IP Multimedia Subsystem service;deriving a Service Control Point from said called party number if saidcall is a request for a connection to said IP Multimedia SubsystemService, where the Service Control Point handles said call; queryingsaid Service Control Point for data associated with said call;processing said data in order to create a Session Initiation ProtocolInvite message; and sending said Session Initiation Protocol Invitemessage to a Call Session Control Function.
 2. The method according toclaim 1, wherein said called party number comprises: a country code ofan operator of said IP Multimedia Subsystem Service; a nationaldestination code for said operator; an address of said Media GatewayControl Function; and a correlation identifier identifying an overlayprocess on said Service Control Point.
 3. The method according to claim2, wherein said testing if said call initiation indicates that the callis a call to said IP Multimedia Service comprises: searching in a listfor an entry resembling a combination of said address of said MediaGateway Control Function and said correlation identifier.
 4. The methodaccording to claim 1, wherein said Service Control Point is queriedusing Simple Object Access Protocol (SOAP).
 5. The method according toclaim 1, wherein said call initiation message comprises an ISDN UserPart Initial Address Message.
 6. The method according to claim 1,wherein said data comprises at least one of: an original dialed numberdialed at said calling party's terminal; a calling party numberassociated with said call; location information of said calling party; acalled party number associated with said called party; locationinformation of a called party associated with said called party number;an International Mobile Subscriber Identification (IMSI); anInternational Mobile Equipment Identity (IMEI); and a subscriber stateassociated with said called party.
 7. A method of executing a service ina Service Control Point for establishing a call between a callingparty's terminal served by a Switching Node and an IP MultimediaService, said method comprising: receiving an IN service trigger fromsaid Switching Node, said IN service trigger comprising data associatedwith said call; storing said data; start up an overlay process for saidcall; allocating a correlation identifier identifying said overlayprocess; sending a destination subscriber number to said Switching Node;and on receiving a request from a Media Gateway Control Function,providing said data to said Media Gateway Control Function.
 8. Themethod according to claim 7, wherein said destination subscriber numbercomprises: a country code of an operator of said IP Multimedia SubsystemService; a national destination code for said operator; an address ofsaid Media Gateway Control Function; a correlation identifieridentifying said overlay process on said Service Control Point.
 9. Themethod according to claim 7, wherein said IN service trigger is based onIN subscription information comprising O-CSI, T-CSI, line based triggercriteria or number based trigger criteria.
 10. The method according toclaim 7, wherein it is determined whether said call is from a GSM MAEsubscriber by virtue of the value of a service key in a CAP Initial DPoperation.
 11. The method according to claim 7, said method furthercomprising: routing said call from said Switching Node to said MediaGateway Control Function in said IP Multimedia Service network.
 12. Themethod according to claim 7, wherein said data comprises at least oneof: an original dialed number dialed at said calling party's terminal; acalling party number associated with said call; location information ofsaid calling party; location information of a called party associatedwith said called party number; an International Mobile SubscriberIdentification (IMSI); an International Mobile Equipment Identity(IMEI); a subscriber state associated with said called party.